I am fairly new to the AS ruleset, and we had a situation on my local table: A squad of BA made an AM attack against a mech. Now, if i am not mistaken, they share the same space (we were using Hex Map rules). How the Mech can attack the attacking BA (or infantry if it is the case). Making a physical attack? Can they use their Short Range attack? Can another mech attack the BA/Infantry?

Stacking (of non-transported units) is not legal in Alpha Strike, even if using hex maps. p43 states the damage of an anti-'mech attack. It would be the short range damage value.Yes, there is no restriction on other units attacking a battle armor that made an anti-'mech attack. There's no actual swarming in Alpha Strike. Anti-'Mech is just a special physical attack.